Themed concepts: Pandemonium
My other themed concepts: Dream Realm Tartown
At beginning there was nothing. Then the first universe was born. However, there were no right or wrong, no justice or crime. At beginning there were only chaos and out of it stars were born. Then planets, moons, living creatures and everything else. That's how it usually happens, though.
There are many alternative universes. Some are connected, some are not. There are many different universes like the Void or Monolith. But, among many different parallel universe there is one quite different than any other. You may wonder why, what is so different there. Well, answer is more bizarre than you may think. It is universe made out of pure absolute evil. Yes, this universe is made out of evil. Evil universe doesn't have planets, stars or anything like that. Instead, it is giant ball of negative energy and corrupted magic. No one knows if it was always like that or something changed this universe into such thing. Yet, it is know that corruption in this places is beyond extreme. Everyone and everything who visit this place is transformed into pure evil. Formless world of malevolent power. The void can't even be compared to this place, because it is like comparing a rabbit to a dragon. Condensed and squeezed into ball of boiling evil -- this is the darkest place in the existence.
So, how evil universe is connected to Runeterra? Answer is -- Pandemonium. It is the palace of demons and evil. Temple of darkness and city of the damned. There is many names it can be called, but most correct would be -- gateway to and for pure evil. It was built in alternative future where demons invaded Runeterra and took over whole planet. In this alternative future humanity lost Third Rune War and was enslaved by demons. Demons used humanity's rune magic, their bodies and souls to build Pandemonium. Many was sacrificed for rituals, other was just used as living paint for the walls. Foundation of human bones, walls of human skin, floors and ceiling of human flesh. Rivers of blood were flowing out of this place in order for to be built. Souls were bound to this place like flesh gets bound to red hot iron.
Inside of Pandemonium demons created a magic device called Infinity Core. This device was made to absorb suffering of human souls, pain and anguish of anyone unlucky to work and die there. It purpose was to gather enough power to open a gateway to the universe of pure evil. Slowly, little by little, demons were closing on their dream. Finally, day have come to open the gateway. 10 humans and 20 demons were present in Pandemonium when Infinity Core was opened. To their surprise this demonic "masterpiece" was much more powerful than anybody would have expected. Infinity Core sent whole temple into that universe. Yes, whole Pandemonium with all 30 residents was sent into universe of evil. No one really knows how much time Pandemonium spent in that place, because that place is without time. Only corruption and evil existed there, so it never needed time.
Infinity Core was activated once again, but this time Pandemonium was sent back in time and appeared in the Valoran. More precisely -- in a massive cave deep bellow Valoran. Pandemonium -- massive castle which traveled into universe of pure absolute evil and back. Travels like that changes thing, twist and corrupt them. Vile and dark place was corrupted even more, but into what...
Pandemonium: city of evil. Current number of inhabitants: unknown. Current location: beneath Valoran. Purpose: open Infinity Core once again.